A viewfinder in general is a system that allows the photographer to view the subject or scene before the photo is actually taken. Such a preview of the photo is essential to find the perfect perspective, composition and to determine proper focus.
Over the past 200 years of photography, several different implementations of viewfinders have evolved. One type of viewfinder that was very common on cameras from the 1960s (such as Polaroid cameras or Canon's Canonet series of cameras) was an optical viewfinder that had a separate opening in the case to accomodate some viewfinder lenses. These viewfinder lenses were completely separate from the actual photo-taking lens of the camera. Therefore, they did not capture any light from the main photographic lens, and therefore did not preview the scene in the perspective of the photo-taking lens. They rather tried to imitate the field of view the photo-taking lens would produce. In fact, however, these separate viewfinders actually quite often suffered from parallax error.
A parallax error occurs when an object placed in front of a background is seen from different positions. The slightly different angle inherent with another position will sometimes result in a very different impression of the scene. Applying this principle to photography, it means that the scene may actually look pleasing on the separate viewfinder but the final image doesn’t exactly turn out as expected.
You can easily comprehend the parallax error if you raise one arm and turn up a thumb. Then, close the right eye and look at the composition of background and your thumb representing a subject to be photograhed. Then, look at the scene with the right eye only while not changing the position of your arm and thumb. You will notice that the composition of background elements and your thumb has changed, eventually resulting in background elements to be covered by the foreground or large gaps between the subject and other objects.
Another challenge of separate viewfinders was that they often had one focal length only. If the main photographic lens was a zoom lens that was used to change the focal length, the separate viewfinder not longer provided a realistic preview. Some improved versions of separate viewfinders had parallax-corrections and features to align with the focal length of the photo-taking lens. However, there were more limitations that could not be corrected in a reasonable way, such as the lack of previewing the scene with photographic filters applied to the main lens.
In digital single lens reflex (DSLR) cameras, no additional optical device is used for the viewfinder, hence the name single lens. A viewfinder in a DSLR camera includes various units of lenses, prisms and displays. As described in the overview, incident light from the main photographic lens is being deflected towards the viewfinder optics by the reflex mirror when the camera is in its idle state.
This use of light from the photo-taking lens is the reason why DSLR cameras do not suffer from parallax error and the photographer can view a scene that does not differ from the scene the sensor will record. DSLR cameras also have a number of very sophisticated auxiliary devices incorporated into the viewfinder unit. The following sub-chapters will explain each of these in great detail.
The images above show the detailed paths of light for some bundles of rays from a photo scene. The white bundle of light is emitted from the center point of that photo scene. The cyan and green bundles of light are emitted from the lateral edges of that scene.
While the mirror is in its lower position, light gets deflected to the focusing screen where it forms an image. The translucent focusing screen emits that light from its top surface and projects it through the condenser lens into the pentaprism. In there, the image gets flipped vertically and horizontally, and is then directed through the eyepiece.
Although a DSLR camera uses the light from the main photographic lens to generate a preview image in the viewfinder, you are not actually looking through the photographic lens when holding the viewfinder at your eye. When you look through the viewfinder, the image you see has been formed on a plastic screen inside the viewfinder assembly, called the focusing screen. The focusing screen is a translucent thin plate that has a very fine-grained matte surface on the lower side.
This matte surface is where the light rays from the photo-taking lens (after being redirected by the primary mirror) form an image. In the past, this focusing screen was often made of glass that has been roughened on one side by sandblasting or grinding, hence the focusing screen is also referred to as ground glass. Today, the matte surface is usually created by laser-etching one surface of the plate, like on Canon's Laser-Matte Focusing Screens.
Looking at some light rays will give a deeper understanding of the focusing screen's functional principle. The diagram shows a conceptional version of a viewfinder with no focusing screen installed. For reasons of simplification, the reflex mirror, pentaprism, viewfinder lenses and other components are not shown in this illustration. Therefore, no reflections are shown, and all components are arranged in a linear way.
The opening on the right represents the viewfinder's eyepiece where a photographer looks at the preview image. With no focusing screen installed, light rays from the photo-taking lens would converge to image points but would then spread again in the same direction. Therefore, light from the outer regions of the image would not reach the eyepiece. A photographer would only be able to see a very small circular part of the viewfinder image in the center but not the complete preview image.
In addition, with no focusing screen, proper focus could be checked or verified through the viewfinder. This is because an out-of-focus viewfinder image would not show as blur but it would move in front of the focal plane or behind it. This slight change in position of the viewfinder image would not be visible to the photographer because the human eye's lens easily adjusts to any distance between the viewfinder image and the retina (the eyes 'sensor').
In order to produce a complete preview image, a focusing screen is placed inside the viewfinder. For the viewfinder to give an accurate representation of the image to be recorded on the camera's image sensor, the focusing screen has to be placed exactly in the primary image formation plane so that it is at the same distance from the photo-taking lens as the image sensor.
The diagram shows how the focusing screen works: The matte screen is an optical diffuser that allows for a dispersion of light rays. This dispersion gives some rays of light a new direction and ensures that even the light rays coming from the outer edges will reach the eyepiece. The image created on the focusing screen acts like a new subject that the photographer can directly view and assess. In addition, with this configuration an incorrectly focused photo-taking lens would show a blurred subject on the focusing screen.
The two functions of a focusing screen can be summarized as follows: 1) It causes a diffusion of light so that even light rays from the outer image points can reach the eyepiece. 2) It allows the photographer to assess the image as it will be recorded by the image sensor (including composition, bright spots and shadows, and correct focus).
It should be noted that the image as seen through the eyepiece is still relatively dark and does not have the intensity that the image sensor will receive. This can easily be seen in the diagram: Due to the diffusion of light, the image points on the focusing screen emit light in such a wide angle that most of the light is blocked by some internal surfaces. To improve the viefinder brightness, a condenser lens is typically installed next to the focusing screen.
Over the past decades, Canon has introduced a variety of focusing screens. Most of these have certain helper guidelines or focusing aids engraved. While split-prisms are very useful for manual focusing, screens with scale lines are often used for macro photography and screens with parallel grid lines are ideal for architecture photography. The illustration shows four of those focusing screens. The concentric rings of the embedded Fresnel lens are much smaller and tighter in reality and are therefore invisible to the eye.
This is a description of the viewfinder's condenser lens. There is also a condenser lens in the phase detection sensor array, which is described in the autofocus chapter.
The introduction of a condenser lens into the viewfinder brings some noticeable improvements to the brightness of the preview image. A condenser lens is a converging lens that reduces the angle at which the light rays diverge after being emitted from the focusing screen. This aims the bundles of light more directly at the eyepiece, and therefore reduces the proportion of light that is lost by hitting internal walls. The result is a viewfinder with a brighter preview image, especially in the edge regions. One disadvantage of a traditional glass condenser lens is, however, that it may contribute quite significantly to the overall weight of the camera and to the size of the viewfinder apparatus. For that reason, today's DSLR cameras do not use glass condenser lenses but have Fresnel lenses installed.
A Fresnel lens is a type of lens with a very characteristic shape but the same refractive properties like a conventional glass lens. Every conventional lens has some material inside that does not contribute to the refraction, and therefore it can be removed without affecting the overall functionality of the unit. This gives a Fresnel lens its unique shape, a flat body that has concentric rings when seen from the front. These thin lenses are also called diffractive lenses, and they are typically composed by a plastic substrate in which the concentric grooves are etched from one side. The diagram shows how the Fresnel lens is much smaller than the conventional glass lens while maintaining an identical refractive power than the conventional unit. Two significant advantages of Fresnel lenses are their much reduced weight and their ultra compact design.
In the viewfinder, the Fresnel lens is either attached to the shiny side (pentaprism side) of the focusing screen as a separate piece, or it can be directly integrated in the focusing screen. It can be seen in the diagram that the Fresnel lens has an identical effect on the viewfinder brightness by narrowing down the light bundles that get emitted from the focusing screen towards the eyepiece.
A pentaprism is a key element that ensures the correct orientation of the preview image in the viewfinder. Another requirement for the pentaprism is that it reflects light with highest accuracy and without introducing any type of distortion. The name results from its five-sided shape and its primary function as a reflecting prism. Although it has five sides, there are only two sides used for the internal reflection of the image while two sides are required to let light in and out. One last side is not optically used but is kept flat for the sake of compact size.
In a regular triangular prism, light can enter on one side (called face) but finally exits the prism on another face. In a reflecting prism, light will not be allowed to exit certain faces but rather is reflected two times and stays inside the prism until it can finally exit one face. The reflections inside the prism are not caused by total internal reflection that would occur if the beams were incident at an angle less than the minimum angle for total internal reflection (critical angle). Instead, the two faces are coated to provide mirror surfaces. The diagram shows how the incident beam reflects inside a regular pentaprism twice. Note that this type of pentaprism is not used in photographic cameras but was only used in this article to explain the basic principle.
As the pentaprism in a DSLR camera faces the image that is produced on the focusing screen, there is one problem. The camera lens produces an image that is both vertically and horizontally reversed. By the reflection of the reflex mirror, this image gets re-inverted vertically, leaving an image horizontally reversed. For the viewfinder to display a correctly oriented preview, the image needs to be reflected left-to-right as the pentaprism transmits the image. This horizontal inversion is done by replacing one of the reflective faces of a standard pentaprism with a section shaped like a roof. That roof is not an additional component but rather describes the shape of the cut: Two additional coated surfaces are angled towards each other in a 90° angle. It is the roof which horizontally reverses the image back to normal when reflecting the image. The diagram shows both features of a roof pentaprism, the deflection of the image and the correction of its orientation.
Most entry-level DSLRs are equipped with pentamirrors. These pentamirrors are not made of solid glass but a hollow case with a similar geometry than a pentaprism but with thin mirrors applied inside. The advantage of pentamirrors is that they are lightweight and inexpensive, but they typically do not provide the same brightness of viewfinder images known by pentaprisms. This can be a critical factor for manual focusing under some low-light conditions. Given the fact that they consist of a small chamber, there is also the risk of dust and moisture entering if no perfect sealing is applied, degrading the mirror surfaces over time. Advanced and pro-level DSLRs typically use pentaprisms in an all-glass design with specially coated faces. They are more expensive and have a higher weight than pentamirrors, but they deliver more light to the eyepiece thanks to their more efficient reflections.
There are different viewfinder coverage ratios among DSLRs. The coverage ratio describes how much of the final image that the camera will record can also be previewed through the viewfinder. While several components within the viewfinder contribute to the coverage ratio, it is mainly the dimension of the pentaprism that really determines whether the viewfinder shows a complete preview image or not. This is because a size reduction of some pentaprism faces will result in a considerable reduction in weight, overall viewfinder size and production cost. For that reason, viewfinder coverage is also dependent on the camera model. While a Canon EOS 60D (mid-range camera) only provides a viewfinder coverage of 96%, the Canon EOS 7D (advanced-level) provides a viewfinder coverage of 100%. This means that the viewfinder of a 60D does not show 4% of the preview image's edge regions while the 7D shows an uncropped preview of the subject. This does not sound like a real difference, but it can require post-processing if an image suddenly shows unexpected elements in the edge regions that have not been seen on-camera.
The eyepiece represents the optical system at the end of the viewfinder’s optical path designed for the photographer to actually see and inspect the viewfinder image. It is an opening at the back of the camera where light from the viewfinder optics exits the camera to enter the photographer’s eye. There is usually an interchangeable eyecup made of soft rubber material applied to the eyepiece, providing for a comfortable contact between the camera and the face. The eyepiece not only consists of the visible opening at the back of the camera, but also includes lenses various convex and concave lenses. These lenses can be adjusted for nearsightedness and farsightedness of the photographer, referred to as dioptric adjustment. Furthermore, these dioptric adjustments are also often required for eyeglass wearers. Depending on the camera model, a dioptric adjustment dial is attached close to the eyepiece. It should be noted that diopter adjustments will never have an effect on the (auto-)focusing capability of the camera as the viewfinder never detects autofocus but only indicates correct focus situations. The diopter adjustment only affects how an eye perceives the preview image.